Shattering Glass by Gail Giles

Simon Glass was easy to hate. He was the geek of the school. Weak, smart, fat. Rob was the king of the school. Gorgeous, smart, charming, and mysterious. All of Rob's posses hate him, for all different reasons. That is until Rob takes him under his wing to prove his power. His plan is to make Simon the Prince Charming of the school. Rob and his followers take him shopping, get his hair styled, and teach him how to behave. Simon is reluctant to become said Prince Charming and starts showing a dark and cruel side, and does the unthinkable -- he defies Rob. Soon, as the manipulative person Rob is, he starts showing a dark side of his own and extracts painful sacrifices and uneasy moral compromises. And what once was a plan to prove power, becomes a parade of violence and death.